1. Technical Field
The present disclosure relates to acoustic devices, acoustic systems using the same, and method for generating sound waves, particularly, to a carbon nanotube based thermoacoustic device, an acoustic system using the same, and method for generating sound waves using the thermoacoustic effect.
2. Description of Related Art
An acoustic device generally includes a signal device and a sound wave generator. The signal device inputs electric signals into the sound wave generator. The sound wave generator receives the electric signals and then transforms them into sounds. The sound wave generator is usually a loudspeaker that can emit sound audible to humans.
There are different types of loudspeakers that can be categorized according by their working principles, such as electro-dynamic loudspeakers, electromagnetic loudspeakers, electrostatic loudspeakers and piezoelectric loudspeakers. However, the various types ultimately use mechanical vibration to produce sound waves, in other words they all achieve “electro-mechanical-acoustic” conversion. Among the various types, the electro-dynamic loudspeakers are most widely used.
Referring to FIG. 36, an electro-dynamic loudspeaker 100, according to the prior art, typically includes a voice coil 102, a magnet 104 and a cone 106. The voice coil 102 is an electrical conductor, and is placed in the magnetic field of the magnet 104. By applying an electrical current to the voice coil 102, a mechanical vibration of the cone 106 is produced due to the interaction between the electromagnetic field produced by the voice coil 102 and the magnetic field of the magnets 104, thus producing sound waves by kinetically pushing the air. The cone 106 will reproduce the sound pressure waves, corresponding to the original input signal.
However, the structure of the electric-powered loudspeaker 100 is dependent on magnetic fields and often weighty magnets. The structure of the electric-dynamic loudspeaker 100 is complicated. The magnet 104 of the electric-dynamic loudspeaker 100 may interfere or even destroy other electrical devices near the loudspeaker 100. Further, the basic working condition of the electric-dynamic loudspeaker 100 is the electrical signal. However, in some conditions, the electrical signal may not available or desired.
Thermoacoustic effect is a conversion between heat and acoustic signals. The thermoacoustic effect is distinct from the mechanism of the conventional loudspeaker, which the pressure waves are created by the mechanical movement of the diaphragm. When signals are inputted into a thermoacoustic element, heating is produced in the thermoacoustic element according to the variations of the signal and/or signal strength. Heat is propagated into surrounding medium. The heating of the medium causes thermal expansion and produces pressure waves in the surrounding medium, resulting in sound wave generation. Such an acoustic effect induced by temperature waves is commonly called “the thermoacoustic effect”.
A thermophone based on the thermoacoustic effect was created by H. D. Arnold and I. B. Crandall (H. D. Arnold and I. B. Crandall, “The thermophone as a precision source of sound”, Phys. Rev. 10, pp 22-38 (1917)). They used platinum strip with a thickness of 7×10−5 cm as a thermoacoustic element. The heat capacity per unit area of the platinum strip with the thickness of 7×10−5 cm is 2×10−4 J/cm2·K. However, the thermophone adopting the platinum strip, listened to the open air, sounds extremely weak because the heat capacity per unit area of the platinum strip is too high.
The photoacoustic effect is a kind of the thermoacoustic effect and a conversion between light and acoustic signals due to absorption and localized thermal excitation. When rapid pulses of light are incident on a sample of matter, the light can be absorbed and the resulting energy will then be radiated as heat. This heat causes detectable sound signals due to pressure variation in the surrounding (i.e., environmental) medium. The photoacoustic effect was first discovered by Alexander Graham Bell (Bell, A. G: “Selenium and the Photophone”, Nature, September 1880).
At present, photoacoustic effect is widely used in the field of material analysis. For example, photoacoustic spectrometers and photoacoustic microscopes based on the photoacoustic effect are widely used in field of material analysis. A known photoacoustic spectrum device generally includes a light source such as a laser, a sealed sample room, and a signal detector such as a microphone. A sample such as a gas, liquid, or solid is disposed in the sealed sample room. The laser is irradiated on the sample. The sample emits sound pressure due to the photoacoustic effect. Different materials have different maximum absorption at different frequency of laser. The microphone detects the maximum absorption. However, most of the sound pressures are not strong enough to be heard by human ear and must be detected by complicated sensors, and thus the utilization of the photoacoustic effect in loudspeakers is limited.
What is needed, therefore, is to provide an effective thermoacoustic device having a simple lightweight structure without a magnet that is able to produce sound waves without the use of vibration, and able to move and flex without an effect on the sound waves produced.
Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate at least one exemplary embodiment of the present thermoacoustic device, acoustic system, and method for generating sound waves, in at least one form, and such exemplifications are not to be construed as limiting the scope of the invention in any manner.